1. Field of the Invention
The present invention relates to a three-dimensional image capturing device by which a three-dimensional shape of a measurement subject, which is to be measured, is captured by a time-of-flight measurement.
2. Description of the Related Art
A three-dimensional measurement using a three-dimensional image capturing device is classified as an active system, in which light, an electric wave or sound is radiated onto a measurement subject, and a passive system in which the light, electric wave or sound is not output. The active system comprises the time-of-flight measurement, a phase detection using a modulated light wave, a triangulation, a moirétopography, and so on, and the passive system comprises a stereo vision system, and so on.
An active system device is bulky in comparison with that of the passive system, since the device requires a laser beam output mechanism. However, the active system device is superior regarding a distance measurement resolution, a measuring time, a measuring range and so on, and thus, despite the bulkiness, the device is utilized in various fields. In a three-dimensional image capturing device, described in “Measurement Science and Technology” (S. Christie et al., vol. 6, p. 1301–1308, 1995), a pulse-modulated laser beam irradiates a measurement subject, and a reflected light beam, which is reflected by the measurement subject, is received by a two-dimensional CCD sensor to which an image intensifier is attached, so that an image signal, corresponding to the reflected light beam, is converted to an electric signal. ON-OFF control of the image intensifier is carried out by a gate pulse, which is synchronized with the pulse radiation of the laser beam. According to the device, since an amount of received light, based on the reflected light beam from the measurement subject, which is positioned far from the device, is less than that of received light based on a reflected light beam from a measurement subject, which is close to the measurement subject, an output corresponding to a distance between the measurement subject and the device can be obtained for each pixel of the CCD.
In a device disclosed in International Publication No. WO 97/01111, light, such as a laser beam, which is pulse-modulated, irradiates a measurement subject, and a reflected light beam, which is reflected by the measurement subject, is received by a two-dimensional CCD sensor which is assembled with a mechanical shutter or an electro-optical shutter formed by a liquid crystal display, so that an image signal corresponding to the reflected light beam is converted to an electric signal. The shutter is controlled at a timing which is different from that of the laser beam, so that distance information of the measurement subject is obtained for each pixel of the CCD.
In the conventional three-dimensional image capturing device of an active system described above, an optical shutter, such as a KDP element, is provided so that an electric charge accumulating operation in the CCD sensor is controlled. However, not only is the optical shutter bulky, but also an electric circuit, which outputs a high voltage to drive the optical shutter, should be provided, and thus the conventional device becomes and remains bulky.
On the other hand, in U.S. Pat. No. 5,081,530, a device, in which an electronic shutter is provided for controlling an electric charge accumulating operation of a CCD sensor, is disclosed. However, an output of the CCD sensor, which is obtained by a single operation of the electronic shutter, is not great enough to sense distance information of the measurement subject.